Tests on a transformer:-

• Tests on a transformer are done to calculate efficiency and regulation of a transformer at any load and at any power factor . Depending upon how the tests are performed the tests on a transformer are divided into two categories. They are

1. Direct loading test:-

• In this test load is directly connected to the secondary of the transformer and various meters are connected to primary and secondary windings of a transformer. Tha advantage of this method is results are accurate. The disadvantage of this method is for large rating transformers suitable load is difficult to obtain in laboratories. Power losses are large during the test. Hence this method is suitable for small transformers and rarely used.

1. Indirect loading test.

• In this method the actual load is not used on transformer. The equivalent circuit parameters of a transformer are determined by conducting two tests on a transformer. They are

1. Open circuit test(O.C Test)
2. Short circuit test(S.C Test)

• Tha advantage of this method is without much power loss the tests are conducted and results are effective.

Short  circuit test(S.C Test):-

• In this test primary winding of the transformer is connected to a.c supply through ammeter & wattmeter. Secondary winding of the transformer is short circuited using thick copper wire. As high voltage side is always low current side, so in this test high votage side is used as primary and low votage side is used as secondary. The circuit diagram of S.C. test is as follows.

• As secondary is short circuited and it has low resistance then when the rated voltage is applied then it causes large current to flow in secondary which causes overheating and burning of the transformer. Thus a low voltage is applied to primary which causes rated current to flow through ammeter. When the primary is connected to the voltage which causes rated current to flow through then the wattmeter measures input power i.e. PSC, ammeter measures input current i.e. ISC and the voltmeter measures voltage i.e. VSC.
• As input applied voltage is low and iron losses are function of applied voltage so iron losses are very small. As the transformer draws full load current the wattmeter reading gives power losses equal to full load copper losses and in S.C. test iron losses are negligible.
• We know

$P_{sc} = V_{sc} I_{sc} cos \phi_{sc}$

• Thus short circuit power factor is given by

$cos \phi_{sc} =\dfrac { P_{sc}} { V_{sc}I_{sc} }$

• Full load copper losses is given by

$P_{sc} = I^2_{sc}R_{1e}$

• Therefore R1e is given by as follows

   

$R_{1e} = \dfrac{W_{sc}}{I^2_{sc}} $

• We know

​$Z_{1e} = \dfrac {V{sc}}{I_{sc}}=\sqrt{R^2_{1e}+X^2_{1e}}$

• we can get X1e as follows

$X_{1e} = \sqrt{Z^2_{1e} - R^2_{1e} }$

• After getting equivalent circuit parameters referred to primary , using transformer ratio K, the equivalent circuit parameters referred to secondary can be calculated.
• If the meters are connected on secondary and primary is short circuited  then S.C. test gives R2e and Z2e . After knowing transformer ratio K we can calulate  R1e and X1e.